1. Field of the Invention
The present invention relates to an excimer laser apparatus, and more particularly to an excimer laser apparatus which serves to pivotally support a rotary shaft of a fan for circulating a laser gas in a laser chamber and to give the rotating force of the rotary shaft by means of a driving motor provided outside the laser chamber.
2. Description of the Related Art
An excimer laser is excited by electron beams or discharging. In case of the electron beams, great output energy is obtained per shot. However, a repetitive operation is hard to perform and the size of an apparatus is increased. In general, an excimer laser apparatus of a discharge excitation type has a laser chamber for completely sealing a laser gas containing a halogen gas of high corrosiveness, in which a gas circulating fan is provided to circulate the laser gas and to feed the gas to a discharge excitation portion. In order to implement a high repetitive excimer laser apparatus, particularly, it is necessary to increase a gas circulation speed in the laser chamber. Thus, the high speed rotation of the gas circulating fan is required. In the excimer laser apparatus, moreover, it has been well known that a laser output light energy is reduced if impurities mix into the laser gas and the amount thereof is increased.
Conventionally, a ball bearing such as a deep groove ball bearing has been used as a bearing of the gas circulating fan. However, it has been hard to use the ball bearing for a long time because a grease necessary for the ball bearing is dried up so that the gas circulating fan is rotated with difficulty or an impurity gas interfering with the operation of the excimer laser apparatus is generated from the grease. In particular, such a tendency has become more remarkable by the implementation of the high-speed rotation of the fan in the corrosive gas atmosphere. Consequently, it has been proposed that a magnetic bearing capable of supporting the rotary shaft of the gas circulating fan in non-contact is applied.
As an example of a magnetic bearing for the excimer laser apparatus, JP-A-10-173259 has disclosed a magnetic bearing of a passive type having such a structure that, in a laser chamber 1 formed by a housing 55 to be also used as a shield partition wall 56, a rotary shaft 30 with a gas circulating fan 4 attached thereto is supported in non-contact by a magnetic bearing 20 including a permanent magnet 22 provided on the rotary shaft 30 side and a permanent magnet 23 provided on a housing 55 side, and the shield partition wall 56 is disposed between a permanent magnet 43 on a magnetic coupling 42 side which is fixed to a shaft 41 of a motor 40 and a permanent magnet 44 provided on the rotary shaft 30 side, as shown in FIG. 4.
In general, the magnetic bearing of a passive type using the permanent magnets 22 and 23 cannot stably support the rotary shaft 30 by itself. Moreover, if the position of the rotary shaft is shifted, it is impossible to control the rotary shaft into a proper position. With such a structure, the shield partition wall 56 provided between the rotary shaft 30 and the magnetic coupling 42 is non-magnetic. Therefore, even if non-magnetic material is used, in case of a metal material such as aluminum or non-magnetic stainless, an eddy current is generated by the rotation of the rotary shaft 30. Due to athe generation of the eddy current, the magnetic force between the permanent magnets 22 and 23 is reduced. In addition, the magnetic force between the permanent magnets 22 and 23 is reduced due to the heat generation of the shield partition wall 56 caused by the generation of the eddy current. Consequently, the holding force of the rotary shaft is reduced to interfere with rotation performance.
As measures to be taken for the drawbacks of such a magnetic bearing of a passive type, U.S. Pat. No. 5,848,089 has disclosed a magnetic bearing of a control type having such a structure that a rotary shaft 130 provided integrally with a gas circulating fan 46 is supported in non-contact by a magnetic bearing comprising a magnetic bearing stator 37 and a magnetic bearing rotor portion 28 in a space sealed with a shield partition wall 36 and is rotated in non-contact through the shield partition wall 36 by means of a brushless DC motor comprising a rotator portion 134 and a stator 140, as shown in FIG. 5.
In the magnetic bearing of a control type, the shield partition wall 36 has such an axially deep and long structure as to include the magnetic bearing rotor portion 28 and the rotor portion 134 of the brushless DC motor. Therefore, the rotary shaft 130 can be supported stably.
In the magnetic bearing of a control type for the conventional excimer laser apparatus, however, the shield partition wall 36 is originally provided in a clearance between the magnetic bearing stator 37 and the magnetic bearing rotor portion 28. Therefore, the thickness of the shield partition wall 36 should be reduced so as to increase magnetic force between the magnetic bearing stator 37 and the magnetic bearing rotor portion 28 as much as possible. In general, a difference in pressure between a laser gas in the laser chamber 1 and an external atmosphere is approximately 3 atm. For this reason, the shield partition wall 36 should have a strength to be resistant to such a difference in pressure. Accordingly, in order to display the performance of the magnetic bearing and to have such a strength as to be fully resistant to the difference in pressure between the laser chamber 1 and the external atmosphere, the shield partition wall 36 having a thin, deep and long structure should be formed of a metal material. Even if the metal material to be used is non-magnetic, an eddy current is generated on the shield partition wall 36 by a rotating magnetic field to generate high heat when the motor is driven in the same manner as in the case of the magnetic bearing of a passive type. Consequently, there has been a problem in that the driving operation of the motor is disadvantageously affected.
It is an object of the invention to eliminate the drawbacks of the conventional example and to provide an excimer laser apparatus capable of preventing heat from being generated on a shield partition wall of a magnetic coupling portion particularly when the rotating force of a motor is transmitted to a rotary shaft in non-contact, and capable of stably supporting and rotating the rotary shaft for a long time without mixing an impurity gas contaminating a laser gas into a sealed space of the rotary shaft.
In order to achieve the above-mentioned problem, the invention provides an excimer laser apparatus, comprising: a laser chamber to be filled with a laser gas containing a halogen gas; a discharge excitation portion disposed in the laser chamber; a rotary shaft attached with a gas circulating fan for circulating the laser gas in the laser chamber; a magnetic bearing portion supporting the rotary shaft in non-contact; a driving motor rotating the gas circulating fan through the rotary shaft disposed outside the laser chamber; and a magnetic coupling portion for transmitting a rotating force of the driving motor to the rotary shaft in non-contact. A shield partition wall is disposed in a clearance between a stator portion of the magnetic bearing portion and a rotor portion of the magnetic bearing portion fixed to the rotary shaft and a clearance between a driving side magnet of the magnetic coupling portion and a driven side magnet fixed to the rotary shaft, and constructs a part of a side wall of the laser chamber to seal a space including the rotary shaft and connected to the laser chamber. The shield partition wall is formed of metal in the magnetic bearing portion and ceramics in the magnetic coupling portion.
According to the invention, thus, the shield partition wall of the magnetic coupling portion is formed of ceramics. Therefore, when the rotary shaft is rotated by the driving motor, a magnetic field does not cause the generation of high heat and a reduction in magnetic force over the shield partition wall of the magnetic coupling portion. Therefore, it is possible to prevent the motor driving operation from being disadvantageously affected.
Moreover, the shield partition wall of the magnetic bearing portion is formed of metal. Therefore, the thickness of the partition wall can be more reduced as compared with ceramics and a spacing between the rotor portion on the rotary shaft side and the stator portion provided outside the partition wall can be reduced. Furthermore, it is possible to obtain such a structure that a thin partition wall is fully resistant to a difference in pressure inside and outside the chamber.
With such a sealing structure as to hold the rotary shaft in non-contact, the shield partition wall constituted by the portion formed of ceramics and the portion formed of metal can completely perform the sealing. Thus, it is possible to completely prevent the halogen gas in the sealed space from being contaminated by impurities generated on the magnetic bearing, the driving motor or the rotary shaft sensor.